Can anyone explain why with B-spline curves (page 13 of SMEG): "P(t) is validly defined for t_min<t<t_max where t_min = t_k and t_max = t_(n+2)"? Why can't the curve be evaluated at t_min=t_1 for example, and what has n+2 got to do with anything? Cheers for any help, Jamie